The composite fuels based on ammonium perchlorate (AP)/aluminum (Al) now used as solid fuels for rockets have a high power, good processability, good mechanical characteristics and a flexible adjustable burn-up or burn-off behavior.
As a result of the use of AP or Al, said fuel types have a strong primary or secondary signature through Al.sub.2 O.sub.3 or HCl in the exhaust gas. However, in the case of use on carrier-bound and field-bound weapon systems, the signature constitutes a serious disadvantage, because launch ramps and sites can be easily located by a smoke plume which can be seen from afar. A further disadvantage is the corrosive action of the exhaust gases.
In addition to AP/Al composite fuels, homogeneous double base fuel systems (DB) based on nitrocellulose (NC) and nitroglycerin (NG) have long been known and described in detail. DB fuels have a relatively weak signature, but have only limited power and unsatisfactory mechanical characteristics (thermoplastics).
In order to eliminate the aforementioned disadvantages of AP/Al composite fuels (strong signature and corrosive exhaust gases) or DB fuels (low power/poor mechanical characteristics), for a considerable time development has been taking place of alternative fuel systems with-energy components burning in smokeless manner and including the following:
Energy carriers: PA0 Inert Plasticizers: PA0 High-energy plasticizers: PA0 Inert binder:
Nitramine compounds, e.g. octogen, hexogen, nitroguanidine, pentraeythritol tetranitrate, tetryl, guanidine nitrate, triaminoguanidine nitrate, triaminotrinitrobenzene, ammonium nitrate, etc. PA1 e.g. glycerol triacetate, dibutyl phthalate. PA1 nitroglycerin (NB), butane triol trinitrate (BTTN), trimethylol ethane trinitrate (TMETN), diethylene glycol dinitrate (DEGDN), bis-dinitropropylformal/acetal (BDNPF/A), etc. PA1 e.g. polyester polyurethane elastomers, polyether polyurethane elastomers, polybutadiene polyurethane elastomers, etc.
The practical usability of the above fuel systems, particularly those containing nitramine, has hitherto failed as a result of the inadequate burn-off rates and the excessively high pressure exponent. The pressure exponent is a measure for the change in the burn-off rate as a function of the system pressure according to the formula r=ap.sup.n (in which r=the burn-off rate, p=the system pressure, a=const.). A reduction in the pressure exponent was noted in the case of DB fuels with nitramine content below 50% and inert polyurethane binders, as well as with additives constituted by heavy metal salts and carbon black. However, the burn-off rate remained at low values. As a result of the unfavorable mechanical characteristics and the poor thermoplastic processibility, the spectrum of characteristics is so unfavorable that these fuels have not been used in practice.
What is desired is a very low pressure exponent, so that in the case of any system pressure there is an identical and high burn-off rate.
When using inert binder systems, burn-off-moderating additives have no significant influence on the pressure exponent. Attempts have been made of late to replace inert binder systems (e.g. polyester polyurethanes) by azide group-containing binder systems, which lead to a power increase. These binders have a polyether-like or polyester-like chain structure containing energy-rich azide groups in the side chain. An example of an azide group-containing binder is a glycidyl azidodiol with the following structural unit: ##STR1## which can be cured with di- or triisocyanates (e.g. hexametholene diisocyanate) to elastomers (GAP). As GAP has a positive enthalpy of formation, solid fuels with this binder have higher power characteristics than those with inert binder systems. However, as in the case of standard formulations with inert binders, the pressure exponent of this fuel formation is much too high (n&gt;0.8).